The present invention concerns an electrode device comprising an ion selective material, a solid state, inner reference system of a bronze and a contact material, where the reference system mediates the electrochemical correspondence between the ion selective material and the contact material. More specifically, the invention concerns a planar, miniaturised electrode device with a solid state, inner reference system.
In many cases it is interesting to analyse samples for content or presence of different ions. For this purpose, electrode devices often include an ion selective membrane, an inner reference system and a contact material, which is connected with usual measuring equipment together with an external reference electrode. The inner reference system mediates the electrochemical correspondence between the ion selective membrane and the contact material, and it has the purpose of maintaining a stable inner electrochemical potential.
As inner reference system, conventional electrode devices comprise a metal dipped in into an electrolyte liquid. The equilibrium reaction between the metal ion of the reference system and the anion of the electrolyte liquid normally takes place via a solid metal salt of these ions, which salt is deposited on the metal. These types of reference systems containing a liquid are stable and reliable in use, but they have some disadvantages. They are relatively fragile, cannot be used at high temperatures, has a storage stability which is limited by evaporation of the electrolyte liquid, and they also limit the possible methods of manufacture due to the fluid electrolyte and the relatively large volume of electrolyte liquid which is required to achieve a sufficient stability for the system. If the volume of the electrolyte liquid is reduced substantially, the system becomes sensitive to redox active substances-and carbon dioxide. The conventional reference systems are actually unsuitable for miniaturisation. Attempts to reduce or avoid these disadvantages have lead to the development of electrode devices with a solid state, inner reference system.
Certain properties of the reference system are thought to have a stabilising effect on its potential. Thus, the ability of the reference system to take up and liberate electrons across a phase boundary Influences the stability of said reference system. The uptake and liberation of electrons typically take place between two phases in the system or between the system and the surroundings and result in a so-called exchange current. It also promotes the stability, if the reaction(s) implicating the uptake/liberation of the electrons are reversible and occur to such extent that the magnitude of the exchange current does not have a limiting effect.
In the literature, several solid state, inner reference systems are described. In one type of reference systems, the same metals and salts are used as in the conventional inner reference systems, but the salts are dissolved in hydrocolloids or other highly-viscous materials with water activity. As the conventional reference systems, these types of reference systems will, if miniaturised, be sensitive to redox active substances and to carbon dioxide present. Their use at high temperatures is also limited. Examples of such inner reference systems are described in e.g. U.S. Pat. Nos. 5,911,862, 5,552,032 and 5,041,792.
In another type of reference system, a redox active substance (e.g. a conventional redox pair or a redox polymer) is immobilised in the membrane or between the membrane and the contact material. The contact material often consists of a precious metal, but can also consist of graphite. The redox active groups may be incorporated in the membrane or constitute a layer between the contact material and the membrane. This type of reference system will typically be sensitive to redox active substances present. Examples of such inner reference systems are described in e.g. EP 498 572; U.S. Pat. Nos. 5,286,365; 5,326,452; 4,871,442; 4,981,570; 4,798,664; 4,816,118; 5,139,626; 5,192,417 and EP 927 884. Among these, typical examples of this type of solid state, inner reference system are EP 498 572, wherein a fortiophore is used for complex binding in the membrane of the ion present in the contact material, U.S. Pat. No. 5,286,365, wherein an electrode device is described in which a reference layer comprising a redox pair is placed between the contact material and the membrane, and U.S. Pat. No. 5,326,452 in which the electrode device has a reference system of iron oxide, and where iron oxide is also incorporated in the glass membrane. This electrode device can be prepared by thick film printing.
In a third type of reference system, a solid state reference system is used which possesses both electron conductivity and ion conductivity. This material may typically be a bronze or another metal oxide. In this case, the stable, inner potential is achieved by an exchange current between bronze/metal oxide and the membrane, and it can be further stabilised if two phases are present in the bronzelmetal oxide.
In GB 1 470 558 such electrode device for determining components in solid or fluid metals or alloys is described. It is especially used for determining sodium in such metals or alloys. The reference system of the electrode device comprises a solid state electrolyte of xcex2-aluminium oxide containing the component which is to be determined, or it comprises the component which is to be determined in solid state, covered by the above-mentioned electrolyte for protection of the solid state reference system. The reference system should preferably have two phases. DE 25 38 739 (GB 1 521 964) and GB 1 602 564 disclose electrode devices, which are further developments of the above-mentioned electrode device. In DE 25 38 739 the reference system includes tungsten bronze or mercury amalgam, protected by the above-mentioned electrolyte. Both the reference system and the protection layer must contain the ion which is to be determined. The reference systems may have several different cations incorporated at the same time. Further, it is mentioned that the tungsten bronze may contain other transition metals including vanadium. These electrode devices are sensitive to the oxygen pressure above the reference system. In the electrode device disclosed in GB 1 602 564, the reference system is constituted by two aluminium oxide phases covered with a further layer for fixing the oxygen potential, in order to reduce this oxygen sensitivity. Metal/metal oxide mixtures are suitable for this layer, e.g. of copper, chromium or nickel. Neither the operational stability nor the storage stability are particularly good for the sodium tungsten bronze.
In WO83/03304 (U.S. Pat. No. 4,632,732) a H30-selective glass electrode device with a solid state, inner reference system is described preferably of biphased lithium vanadium bronze. It is mentioned that other lithium bronzes and sodium tungsten bronze may also be suitable. Such a lithium vanadium bronze as well as an electrode device including this must be prepared under anhydrous and oxygen-free conditions. Therefore, in practice, the electrode device will be complicated and expensive to prepare e.g. by thick film printing.
In U.S. Pat. No. 3,853,731 an ion selective glass electrode is described with a solid state reference system of a composite material of silver and silver halide. A paste of silver oxide and a silver salt of a halogen oxy acid fixed to the glass membrane is heated to obtain the composite material of silver and silver halide.
In U.S. Pat. No. 5,122,254 an Na+-selective electrode device is described with a solid state electrolyte membrane containing sodium, zirconium, silicon etc. The solid state reference system consists of e.g. sodium tungsten bronzes, sodium molybdenum bronzes or sodium alloys. These must be biphased compounds. The electrode device can be prepared by thick film printing.
DE 41 12 301 describes a reference electrode comprising an alkali compound in a transition metal oxide, e.g. nickel oxide or cobalt oxide. This is very suitable for gas sensors. WO 95/22050 describes a reference electrode comprising an ion conductive solid state electrolyte, which is in contact with a glass phase.
DE 2 002 676 discloses an electrode device where the membrane consists of an ionic semiconductor, e.g. lanthanum fluoride, for measurement of fluoride. The reference system in such an electrode device may consist of e.g. lead or bismuth in epoxy polymer, which metals form an ionic semiconductor with fluoride.
Electrode devices for detection of gasses are also described. DE 43 24 922 and U.S. Pat. No. 5,112,456 disclose electrode devices for detection of oxygen which are based on various metal/metal oxide mixtures. U.S. Pat. No. 4,861,454 describes an electrode device for detection of oxygen which are based on a redox pair. Electrode devices for detection of CO2 are described in U.S. Pat. No. 5,910,239 and in U.S. Pat. No. 4,839,020. The first is based on stannate/titanate, and the other is of the Severinghaus-type, where the pH-electrode device is based on a reference system comprising a redox pair. In WO 96/12944 a thick film printed CO2-sensor is described comprising copper oxide and titanium oxide.
Various bronzes with intercalated cations can also be used as selective materials in ion selective electrode devices. In such electrode devices, the bronze is both electron conductive and ion conductive, but the potential will also vary. Examples of this type of electrodes are disclosed in e.g. U.S. Pat. No. 3,825,482 and U.S. Pat. No. 3,856,634, wherein sodium tungsten bronze is used as selective electrode material.
Finally, in the report xe2x80x9cSolid state sodium batteriesxe2x80x9d (Steen Skaarup and Keld West, Energiministeriets Forskningsprogram: Energilagring, Journ. No. 2263-407; 1443/85-2; 1443/86-3; 1443/87-4, April 1989), a research program is disclosed. The purpose of this was to characterise some potentially suitable sodium transition metal oxides for rechargeable batteries compared to for example lithium bronzes. Experiments have been carried out with, among others, sodium bronzes of molybdenum, chromium and vanadium with varying molar ratios between sodium, transition metal and oxygen. In the articles xe2x80x9cSolid State Sodium Batteriesxe2x80x9d (S. .Skaarup et al., Proceedings of the International Seminar on Solid State Ionic Devices, edition B. V. R. Chowdari and S. Radhakrishna, Singapore, (1988) 75-86) and xe2x80x9cSolid-State Sodium Cellsxe2x80x94An Alternative To Lithium Cells?xe2x80x9d (K. West et al.,J. Power Sources 26, 1989, pages 341-45) sodium vanadium bronzes are characterised for use in rechargeable batteries.
The previously proposed solid state, inner reference systems for replacement of the conventional inner reference systems containing a liquid still have certain disadvantages. Some systems impart to the electrode device an unsatisfactory stability, others are sensitive to redox active substances and CO2. Some of the previously suggested reference systems impart acceptable properties, but requires expensive raw materials, are complicated to prepare or require preparation and use under controlled conditions. Thus there is still a need for electrode devices comprising a solid state, inner reference system with a stable potential and high sensitivity, where the reference system is further uncomplicated and economical to prepare and can be applied by methods which are suitable for miniaturisation such as thick film printing.
The object of the present invention is to provide an electrode device of the type mentioned in the introduction comprising a solid state, inner reference system which is economical and simple to prepare and at the same time imparts to the electrode device properties as regards for example sensitivity and stability, which are as good or better than the properties of known electrode devices. It is especially an object of the present invention to provide a planar, miniaturised electrode device with a solid state, inner reference system, which can be applied by thick film printing.
In accordance with this and according to the invention, an electrode device is obtained which comprises an ion selective material, a solid state, inner reference system of a bronze and a contact material. The reference system mediates the electrochemical correspondence between the ion selective material and the contact material. This electrode device is characterised in that the reference system comprises sodium vanadium bronze, where sodium is incorporated in the vanadium bronze at such a stoichiometric proportion that the insertion/liberation of sodium is reversible. An xe2x80x9cion selective materialxe2x80x9d is to be understood as a material which constitute a diffusion barrier against the surroundings, but which is sensitive to one or more ions of interest. The term xe2x80x9cinner reference systemxe2x80x9d is to be understood as a system imparting to the electrode device a stable inner potential, as described above. Finally, the term xe2x80x9ccontact materialxe2x80x9d describes a material which is capable of mediating electrical contact between the inner reference system and usual measuring equipment, e.g. through an outer electric conductor. The xe2x80x9celectrochemical correspondencexe2x80x9d signifies the coupling between the ion transport and the electron transport
The design of a preferred embodiment of the electrode device according to the invention is similar to the design of conventional ion selective electrode devices. It comprises an ion selective material aiming to segregate a particular sample from the inner part of the electrode device and at the same time allow the ion/ions in the sample, to which the material is sensitive, to affect the electrode device significantly. The ion selective material is also in contact with the solid state reference system so that an exchange current can be obtained between the ion selective material and the reference system. The solid state reference system is also in contact with the contact material so that it can mediate the electrochemical correspondence between the ion selective material and the contact material. When the electrode device according to the invention is used, the contact material is connected with usual measuring equipment, e.g. via an outer electric conductor. The content of the ion in the sample to be determined by the electrode device is read out on the measuring equipment.
Even though in the electrode device according to the invention the contact both between the ion selective material and the solid state reference system and between the solid state reference system and the contact material are presented as being direct, it must be understood that an electrode device comprising the solid state reference system in indirect contact with the Ion selective material and the contact material, respectively, also will be within the scope of the Invention, if only the solid state reference material is not thereby prevented from functioning as such.
The solid state, inner reference system in the electrode device according to the invention comprises vanadium bronze with sodium incorporated in the lattice of the bronze. The term xe2x80x9cbronzexe2x80x9d as used herein refers to ternary metal oxides with the formula MxTyOz, which are electron conductive, and where T is a transition metal which is capable of being oxidised/reduced, and M is another or several other metals or hydrogen. x is an arbitrary value and represents the content of M per formula unit. y and z indicate the content of T and O respectively in the formula. In such bronzes the transition metal oxide is said to form the host structure and the other metal or hydrogen to be incorporated in this structure.
According to this, the sodium vanadium bronze in the electrode device according to the invention will have the formula NaxVyOz, where the vanadium oxide forms the host structure and sodium is incorporated in this structure. At certain stoichiometric proportions between the three components, it is possible to obtain structures in which sodium can be reversibly inserted and liberated (further described in Faststof-natriumbatterier, Steen Skaarup and Keld West, Energiministeriets Forskningsprogram: Energilagring, Journ. No. 2263-407; 1443/85-2; 1443/86-3; 1443/87-4, April 1989). If this process is reversible, the sodium is said to be intercalated in the bronze, and the process is called an intercalation.
Surprisingly, it has been found that sodium vanadium bronzes with such stoichiometric proportions, that the incorporation of sodium is reversible, are very suitable for solid state, inner reference systems in electrode devices. Such reference system is capable of having electrons and sodium ions reversibly taken up and liberated across a phase boundary, and in this case the phase boundary between the reference system and the ion selective material. Thus the intercalation of sodium is connected with uptake and liberation of electrons. This property gives the bronze the ability to mediate the electrochemical correspondence.
The reference system of the electrode device according to the invention possesses thus surprisingly these potential stabilising properties in defiance of the fact that the sodium vanadium bronze is not biphased, which in the prior art is described to be preferred.
A solid state, inner reference system comprising sodium vanadium bronze has some further advantages. It is not sensitive to humidity, CO2 or redox active substances such as oxygen. The bronze is then substantially insensitive to air humidity. Both the operational stability and the storage stability are excellent which is due to the fact that the system is neither sensitive to oxygen, CO2, humidity nor evaporation of fluid. The use of sodium vanadium bronze as a solid state, inner reference system in the electrode device imparts that the preparation and the application of the reference system can take place under atmospheric air. This, combined with avoiding a liquid reservoir, makes it possible to use new, more simple and more efficient procedures of preparing the electrode devices according to the invention, e.g. thick film printing. The electrode devices according to the invention may be used at high temperatures as they do not comprise a liquid reservoir.
The lattice structure (and thus the unit cell) of sodium vanadium oxides may vary depending on the preparation conditions and the content of sodium. The sodium vanadium bronze for use in reference systems in electrode devices according to the invention are typically prepared by heating a mixture of NaVO3 and V2O5 in a suitable molar ratio between 400 and 800xc2x0 C. depending on the wanted structure. It is advantageous if the mixture of NaVO3 and V2O5 is beforehand ground to a particle size smaller than 1 mm, and preferably smaller than 1 xcexcm. By heating, a thermodynamically stable structure is obtained, which can be cooled to the ambient temperature. After cooling to 150xc2x0 C. or below and under anhydrous and oxygen free conditions a further sodium Incorporation may result in a bronze which is no longer thermodynamically stable, but where the sodium incorporation is however reversible. When using the sodium vanadium bronze for reference systems in electrode devices according to the invention for potentiometric determination of an analyte it is however not particularly advantageous to make this subsequent intercalation.
According to a preferred embodiment of the invention, the reference system comprises thus sodium vanadium bronze of the formula NaxV2O5, which is thermodynamically stable for 0.33 less than xc3x97 less than 0.40. After cooling to 150xc2x0 C. or below and under anhydrous and oxygen free conditions, sodium may however by intercalation vary between x=0.01 and x=1.6 retaining approximately the same lattice structure and thus preserving the reversible conditions. This thermodynamically stable sodium vanadium bronze can be obtained by mixing NaVO3 and V2O5 in a molar ratio of between 2:4 and 2:5 and heating to approximately 650xc2x0 C. until equilibrium after which it can be cooled.
According to another preferred embodiment of the invention the reference system comprises sodium vanadium bronze of the formula (unit cell) Na1+xV3O8. This structure can also be obtained by mixing NaVO3 and V2O5 in a molar ratio of approximately 1:1 and heating to around 700xc2x0 C. until equilibrium is reached. After cooling to 150xc2x0 C. or below and under anhydrous and oxygen free conditions, sodium may however by intercalation vary between x=0 and x=2.2 retaining approximately the same lattice structure and thus preserving the reversible conditions.
Even though the above-mentioned structures are indicated with a formula representing an ideal stoichiometry between vanadium and oxygen, it must be understood that a bronze in which only the major part of the unit cells has this stated stoichiometry, also will fulfil the conditions of reversible incorporation of sodium and thus will also be within the scope of the invention. Accordingly, it is preferred that at least 90-95% of the bronze has a structure according to one of the formulas NaxV2O5 or Na1+xV3O8.
The above-mentioned limits for x are found under atmospheric air. However, it is likely that other limits for x can be found by modifying the preparation conditions and the conditions for use such as changing the oxygen partial pressure. Therefore these limits are just for stating preferred limits and not to limit the scope of the invention, since all sodium vanadium bronzes meeting the condition of reversible incorporation of sodium are suitable for reference systems in electrode devices according to the invention.
The bronze formed can be used as it is for a reference system in an electrode device, or it can be ground to a particle size with a diameter of from 0.001 xcexcm to 100 xcexcm, preferably from 0.1 xcexcm to 10 xcexcm, and particularly from 2 xcexcm to 5 xcexcm. This powder can then be pressed into a suitable shape, or it can be mixed with a suitable binder system to obtain a paste.
According to yet another preferred embodiment of the electrode device according to the invention, the bronze powder is mixed with a suitable binder system to obtain a paste which can be applied by methods suitable for miniaturisation. A suitable binder system is to be understood as a more or less viscous system binding the powder to a uniform mass. Such binder system typically comprises a suitable binder and various solvents and additives. One suitable binder system is the acrylate based S1112, which is accessible from ESL.
The purpose of the binder is to form a matrix for binding the bronze to obtain a reference system in solid form. For such binder systems, suitable binders which can be mentioned are polymers hardening by evaporation of solvent or by chemical reaction such as polyester, polymethacrylates, polyacrylates, butadiene acrylonitrile copolymer, polyvinyl chloride (PVC), polyurethane, polycarbonate, polyoxymethylene, polystyrene, polysiloxanes, epoxides, silicone, cellulose or cellulose derivatives, e.g. cellulose acetate, ethyl cellulose or propyl cellulose, or mixtures hereof.
Solvents and additives are added to give the paste the right application and hardening properties for the method chosen to prepare of the electrode device. They normally disappear or are consumed during hardening. Usable solvents are e.g. carbitol acetate or similar.
The properties of the above-mentioned paste comprising the bronze may be varied depending on the method of preparing the electrode device chosen. The viscosity of the paste before application is preferably from 0.2 Paxc2x7s to 7,500 Paxc2x7s, especially preferred from 2 Paxc2x7s to 750 Paxc2x7s, and for thick film printing advantageously from 100 Paxc2x7s to 650 Paxc2x7s. After hardening, the ratio between sodium vanadium bronze and binder system is preferably from 95:5 to 5:95 by weight, and especially preferred from 80:20 to 20:80 by weight.
The aforementioned reference system is suitable for electrode devices with sensitivity to various ions. The variety of ions which can be detected by the electrode device according to the invention are in principle only limited by which ions it is possible to prepare ion selective materials for.
Till now, it has been mentioned as an important condition for applicability of the electrode devices comprising known reference systems of the bronze type that the bronze should contain the ion to be detected (see e.g. GB 1 470 558; GB 1 521 964; GB 1 602 564 and U.S. Pat. No. 5,122,254). In accordance therewith, another surprising property of the reference system in the electrode device according to the invention is that the reference system is suitable for detecting a variety of ions and not only the ion contained in the reference system (here sodium).
The ion selective material can be any suitable Ion selective material, both of organic and inorganic nature. It should simply be capable of acting as a diffusion barrier towards the surroundings and at the same time be sensitive to one or more ions of interest.
The electrode device according to the invention can thus be used for detection of cations such as H+, Li+, Na+, K+, Rb+, Cs+, NH4+, Mg2+, Ca2+, Sr2+, Ba2+, Ag+, Pb2+, Cd 2+, Ni2+and Co2+. It is also applicable for detection of other positively charged groups, such as the trimethylammonium ion, positively charged amino acids, positively charged nucleic acids or macromolecules comprising these, including pharmaceutical preparations.
According to yet another preferred embodiment of the invention, the electrode device comprises an ion selective material in the form of a membrane prepared from a polymeric material, where the membrane comprises an ionophore for one or more ions of interest and optionally a plasticizer. Among suitable ionophores, crown ethers can be mentioned, such as 18-crown-6 cryptands, for example 2,2,2-cryptand, calixarenes, for example 25,26,27,28-tetrakis(ethoxycarbonylmethoxy)-p-tert.butylcalix[4]arene, cyclic peptides, for example valinomycin and nonactin, noncylic multidentate amides, for example (xe2x88x92)-(R,R)-N,Nxe2x80x2-(Bis(11-ethoxycarbonyl)undecyl)-N,Nxe2x80x2-4,5-tetramethyl-3,6-dioxaoctanediamid (ETH 1001) and aliphatic amines, for example tridodecylamin. Suitable polymeric materials for the membrane are e.g. polyvinyl chloride, polymethacrylates, polyacrylates, silicones, polyesters or polyurethane or mixtures hereof. Among suitable plastlsizers can be mentioned di-octylsebacate and di-octylphthalate.
The contact material may comprise any suitable electron conductive material. Often it will comprise one or more precious metals, such as gold, palladium, platinum, rhodium or iridium and preferably gold or platinum, or mixtures hereof. Other suitable electron conductive materials are graphite or iron, nickel or stainless steel. The electron conductive material can be mixed with another component, such as a binder system having an advantageous effect on the properties of the contact material, both in connection with the preparation and the use of the electrode device.
The contact metal can be used as it is, e.g. as a metal wire, for contact material in an electrode device, or it can be ground to a particle size with diameter of from 0.001 xcexcm to 100 xcexcm, preferably of from 0.1 xcexcm to 10 xcexcm. This powder can then be pressed to a suitable form, or it can be mixed with a suitable binder system for obtaining a paste.
According to a preferred embodiment of the electrode device according to the invention, the contact metal in the form of powder is mixed with a suitable binder system to obtain a paste. Platinum paste is preferably used, such as P2607 (SIKEMA).
The electrode device according to the invention can be prepared as a conventional tubular electrode device or as a planar electrode device, which is provided on a support, the support being cut out in any shape desired. This electrode device is suitable for miniaturisation.
According to yet another preferred embodiment, the electrode device according to the invention is designed as a planar electrode device, which is provided on a support. The support can be made of any suitable material. However, it cannot be electron conductive and it must be able to resist the conditions under which the electrode device is hardened and used. The material usually comprises a ceramic or polymeric material.
Ceramic supports have the advantage that they are thermally, mechanically and chemically stable. If ceramic supports are used in combination with polymeric membranes, it may be necessary to use an adhesive material so that the membrane adheres to the adhesive material and the adhesive material adheres to the support. An example is disclosed in U.S. Pat. No. 5,844,200. Aluminum oxide and fosterite are ceramic materials which are suitable as supports.
Polymeric supports are more economic to use and may result in a better adhesion between polymeric membranes and support, than in the case of ceramic supports. Particularly good adhesion can be obtained if the membrane and the support are based on the same type of polymer. Polymeric supports also give less limitations on possible geometric designs than the ceramic support does. If polymeric supports are used, the hardening and use must often be carried out at lower temperatures. Among polymeric materials which may be suitable as supports can be mentioned polyvinyl chloride, polyester, polyimide(kapton), polymethylmethacrylate or polystyrene.
According to a preferred embodiment, the support can also constitute support for other electrode devices so that several electrode devices can be provided on one and the same support. For example an array of electrode devices sensitive to Ca2+, K+, Na+and H+, respectively, can be provided. Such an array is suitable for simultaneous detection of all of said ions when present in the same sample, e.g. a blood sample.
The solid nature of the inner reference system opens up the possibility of preparing electrode devices according to the invention by new and more economic and efficient methods. Especially it is possible to prepare the electrode devices according to the invention by methods suitable for miniaturisation, such as by thick film printing, drop casting, spray coating or spincoating.
Thick film printing is particularly suitable for application of thin layers and in well-defined shapes, especially on plane surfaces. The thick film printing process resembles very much other printing processes such as screen printing. Briefly, a particular homogeneous paste is pressed through a screen with a suitable fineness and pattern on the surface which is to be covered. The desired pattern of the screen is generally obtained by first applying a photo sensitive emulsion to the screen which closes all the meshes. Then a negative illustrating the desired shape of the print is placed on top of the screen and this is developed resulting in the emulsion being dissolved in exposed areas. After rinsing the screen, it is ready for use. The fineness of the mesh and the thickness of the paste determines the thickness of the layer. The technique is further described in the book Polymer Thick Film (Ken, Gilleo. Edt. New York: Van Nostrand Reinhold, 1996) and in U.S. Pat. No. 5,844,200, in which the preparation of planar, miniaturised electrode devices prepared by thick film printing is disclosed. This technique is suitable for preparation of miniaturised, planar electrode devices and for mass production. Further, it is an advantage if the process can be carried out under atmospheric air.
A preferred embodiment of the electrode device according the invention is thus a planar, miniaturised electrode device prepared by thick film printing. Advantageous properties for such electrode devices are that they only require very small sample volumes, and that the method of preparation is suitable for mass production of electrode devices. If desired, only the contact material and the reference system are applied by thick film printing, after which the ion selective material is applied.
Another method suitable for preparing electrode devices according to the invention is spincoating. For instance a support similar to the above-mentioned can be spincoated with contact material of the type described above. Then a V2O5-xerogel prepared by ion exchange of NaVO3 through a H+-column can be spincoated on the contact material. After drying, the V2O5-xerogel can be impregnated by a NaCl-solution. Na+will spontaneously be absorbed until a composition corresponding to the unit formula Na0.33V2O5 is obtained. A heat treatment at 650xc2x0 C. stabilises the structure, Then a membrane can be dispensed above the bronze layer in the usual way.
A preferred embodiment of the electrode device according to the invention further comprises a reactive material, in which an ion product, to which the ion selective material is sensitive, can be formed from a particular analyte. By xe2x80x9creactive materialxe2x80x9d is meant a material which can be affected by a particular analyte resulting in a physical or chemical change. A particular analyte is any component which is present in a sample and for which it is desired to determine the content or presence. Among such electrode devices can be mentioned biosensors: and electrode devices of the Severinghaus-type.
In such a preferred embodiment of the electrode device according to the invention, the reactive material comprises a biological recognition component, where the recognition of a particular analyte occurs with formation of an ion product. The reactive material may be an integrated part of the membrane, or it may constitute a separate layer in the electrode device. The recognition component can e.g. be an enzyme, such as a hydrolase, drolase, an oxidase, or a reductase, or an antibody or a receptor, and the ion product to which the ion selective material is sensitive, can e.g. be H+or NH4+.
Another example of such a preferred electrode device according to the invention is an electrode device of the Severinghaus-type. The general design of electrode devices of the Severinghaus-type is described in the article xe2x80x9cA Combined Transcutaneous pO2 and pCO2 Electrode With Electrochemical HCO3 Stabilisationxe2x80x9d by John W. Severinghaus, published in Journal of Applied Physiology, volume 51, No. 4, pages 1027-1032, March 1981, and in U.S. Pat. No. 4,836,907.
In electrode devices of this type, a gas which forms an acid or a base when contacted with water, diffuses into a reservoir with a more or less viscous aqueous electrolyte solution as reactive material. The pH-value in this fluid will then change relative to the partial pressure of the gas. If a pH-sensitive (H+-selective) material is placed above the contact material and the reference system, this change can be detected, when at the same time the electrolyte solution is contact with a reference electrode, such as an Ag/AgCl-electrode.
The aqueous electrolyte solution preferably contains chloride ions and bicarbonate ions, for example in the form of KCl and NaHCO3. The aqueous electrolyte may be based on aqueous glycol solutions, e.g. solutions of tetraethylene glycol, glycerol and ethylene glycol. Thickening agents can also be added, such as hydrocolloids, e.g. poly(vinylpyrrolidone), methylcellulose and ethyl cellulose, agar or similar.
Often, the reservoir will be segregated from the sample by a gas permeable diffusion barrier. This may e.g. be a membrane based on silicone, softened PVC or poly(tetrafluoroethylene). Among gasses which can be detected by this type of electrode device, CO2 and NH3 can be mentioned.
The electrode devices according to the invention disclosed above can also be prepared by methods which are suitable for miniaturisation as mentioned in connection with the ion selective electrode devices, and can therefore also be embodied as planar, miniaturised electrode devices.